The invention relates to a coating mixture for application of a polymeric covering to a metallic substrate, the coating mixture containing at least one compound which can undergo free-radical polymerization and at least one photoinitiator. The invention furthermore relates to a process for application of the coating mixture for a polymeric covering in the region of the edge and/or a seam of a metallic substrate and a substrate coated in this manner, in particular a metal strip, metal sheet or a metallic shaped part, and the use thereof.
Metallic strips, sheets and shaped parts are produced in very high piece numbers. Nevertheless, in certain production processes metallic substrates coated with at least one layer of lacquer have an inadequate or even no corrosion protection at cut edges, at edges protected with a dipcoating or at seams, such as e.g. fold seams. To date, no sufficiently corrosion-resistant process capable of rapid coating is suitable for coating with a corrosion-resistant covering which allows protection of the edges and/or the seams.
This requires a very high outlay on plant technology and energy consumption—especially in the case of strip installations which run at a high speed. The speed of strip installations in which curing is to be carried out largely or completely by UV irradiation is currently limited to a speed of up to about 60 or up to about 80 m/min. In the foreseeable future there will be strip installations which will run at coating speeds in the region of up to 160 or even 200 m/min. The investment outlay for this is exceptionally high.
While e.g. the steel sheets for coating of which the mixture described above has been developed have to date usually been chromated to achieve a higher corrosion resistance, there is a move away from this more and more for environmental protection reasons. It is assumed that at least in the automobile industry in the near future practically exclusively metal sheets which have been pretreated without chromates and optionally have also already been lacquered will be employed.
Such non-chromated metallic substrates, that is to say, for example, steel strips or steel sheets, in some cases require, as has now been found, a higher thickness of the polymeric layer applied to them in order to ensure the same corrosion resistance as on chromated sheets. According to the prior art, the dry film layer thickness of the polymeric coating which is applied to the pretreatment coating is often in the range from 10 to 100 μm. Higher demands are then also made on a coating in the region of edges and/or seams.
Conventionally, no e.g. strip-like protection of the edge or seam regions has hitherto been employed. A change in the composition of the polymeric mixtures for the formation e.g. of lacquers for protection of the edges or seams is unavoidable, in order, for example, to achieve a heat resistance up to about 180° C. for drying of an electro-dipcoating which may be applied subsequently and a high resistance to weathering. The adhesive strength of the polymeric covering on the substrate must also usually be higher than customary in order to achieve a high impact resistance. For protection of sharp edges or of edges at seams and their particular adjacent regions, a corrosion protection which is higher than otherwise is conventionally also required. In certain cases it may be necessary also for these high-quality properties already to be achieved with a comparatively thin coating.
Components or sheets with areas which are to be soldered or welded have often been protected beforehand by a polymeric coating, in particular by at least one lacquer layer, such as a primer or clear lacquer. During soldering or welding, these coatings are often severely damaged in the region of the areas exposed to heat during this operation, which are called “soldered points” or “weld seams” respectively in the following, although it should be at least one individual weld point. So-called burning loss occurs. Due to the combustion of polymeric material, troublesome porosity may occur in the soldered or welded region.
Moreover, these areas which have been attacked are often worked mechanically afterwards, in order to even out or smooth the soldered or welded region, the areas with the damaged coating frequently being increased further in size. There is therefore the need for soldered points, weld seams or damaged areas of the coating (so-called “other areas”) to be covered subsequently with a protective coating which is limited in its area and which usually should project a little beyond the damaged areas. Such a sealing of the weld seam or repair sealing is of interest in particular on components of aluminium alloys, magnesium alloys and steel.
Moreover, the steel producers desire in particular coating mixtures which render possible complete treatment and coating in one production unit, without interruption, of the steel or steel sheet subsequently supplied to the further processors. To date, the steel strips and sheets are generally galvanized or, if desired, chromated and subsequently wound on rolls by the producer after the end of the rolling process. The rolls of the metallic strip obtained in this way (coils) are then transported to the coating unit in which the polymer-containing coating is applied. Transportation to the coating unit and the unwinding and winding up of the rolls are undesirable cost factors which are to be avoided. Other types of repair points, e.g. on polymeric coatings which are incomplete, inadequate and/or too thin or/and on reworked areas, could also be protected by this means.
The Applicant has hitherto known no use of a lacquer or a lacquer-like coating in an automated process which is applied to strips, sheets or shaped articles and is intended only for protection of the edge or the seam. In automobile construction or aircraft construction, protection of edges and seams is utilized, in particular, by injection of hollow cavity waxes and the use of sealing compositions or rubber lips.
Coatings e.g. of a phosphating and an electro-dipcoating also are completely inadequate in respect of their corrosion protection. A substitute for a cathodic dipcoating also proves to be no better.
A UV-curable coating mixture which enables pretreatment and coating with a polymer-containing covering to be carried out on a strip, sheet or shaped part in a single production unit (so-called inline process) is desired. For this, particular requirements are imposed on the curability of the coating mixture. Galvanizing of a steel sheet is generally carried out at speeds from approx. 60 m/min. In order to ensure a problem-free process without intermediate storage, coating, including curing, must also be carried out at such speeds. Coating mixtures with which the layer obtained has a sufficient hardness and which, where appropriate, allow coating at these high speeds are therefore desired.